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Method for Improvement of Wiring Strength in Flexible Circuit Assemblies

Publishing Venue

Abstract

Disclosed is a method for improvement of wiring strength in flexible circuit assemblies.

Country

Undisclosed

Language

English (United States)

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Flexible circuit assemblies, by nature, are designed to allow the circuit to flex and bend in the 'z' direction, if the traces run in the x-y plane. However, as wiring densities increase, the trace thicknesses are getting increasingly thinner in all dimensions. This creates a less mechanically robust interface at the trace-to-via junction.

The trace-to-via junction is a surface discontinuity, and the traditional design of this

junction is a

90-degree angle as indicated below. This creates a mechanical stress riser at the 90-degree angle. It has been proven (and is supported theoretically) that cracks in the wiring layers originate at this junction. A mechanical stress riser exists when a geometry exhibits a sharp transition, such as a corner, fold, or crack. Corners and folds lead to cracks (for stress relief) and cracks themselves are stress risers and lend themselves easily to crack propagation [1, 2].

This disclosure offers a solution to this stress riser by changing the geometry of the trace-to-via junction. It has been demonstrated with this enhancement to the trace to via junction, cracks, and thus electrical discontinuities, are greatly reduced. Even with the assembly exposed to thermo-mechanical stresses, cracks do not originate and propagate (see Figure 1).

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This disclosure addresses the stress riser of the trace-to-via interface by altering the geometry of the interface.

As shown in Figure 2, change artwork to widen via to 2-D "delta" at via interface.

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As shown in Figure 3, add plating feature to trace at via during laminating/pressing process.

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As shown in Figures 4a and 4b, "fatten" trace to via interface in x-y-z direction throug...